But the Prison Officers Association, whose National Executive Committee sanctioned the walkout at 7am on Thursday, insists it had to make its message heard.

It announced the walkout at the last minute to prevent the Government obtaining an injunction to prevent it going ahead.

It is opposing Government plans to link the age at which prison officers can retire to the state pension, on the grounds that they cannot be expected to deal with sometimes violent inmates at the age of 68.

Steve Gillan, general secretary of the union, said in a statement: “The POA has submitted a case to Government to support our view that it is unrealistic for Prison Officers to be automatically linked to the state pension age, which will ultimately rise to 68 years of age.

"Unfortunately, it has fallen on deaf ears and Prison Officers have no other option but to protest to gain public attention.”

The only other protest action taken by the POA was in 2007, when an estimated 20,000 officers went out on strike over pay.

Michael Spurr, chief executive of the National Offender Management Service, said: "I am extremely disappointed that the Prison Officers Association (POA) has taken this unlawful action. We have implemented our contingency plans, and our priority is to protect the public and ensure that prisons remain safe and secure.

"In 2007, the POA agreed that the normal pension age for new prison officers would be 65, in line with all other civil servants. The Government has been in constructive discussions with the POA about further pension reform and it is deeply regrettable that this action has been taken now."

Meanwhile thousands of bus workers are to vote on strikes in a row over payments during the Olympic Games.

Unite said it had given notice to 21 bus companies in London of its intention to ballot up to 21,000 members in the capital from next week.

The union has called for a £500 payment to recognise the "massive increase" in workload during the Games this summer, saying that 800,000 extra passengers are expected to travel on buses.

Unions said today's strike was being solidly supported.

Union leaders predicted that up to 400,000 workers, ranging from police officers and immigration staff to lecturers and job advisers, will be involved in a wave of demonstrations, fuelled by ministers making clear in yesterday's Queen's Speech that they are pressing ahead with their controversial reforms.

The Public and Commercial Services union (PCS) said early signs from picket lines showed solid support for the strike, the third major walkout by public sector employees in the past six months in protest at the pension reforms.

The PCS said business at the Welsh Assembly was hit after Labour and Plaid Cymru members pledged not to cross the picket lines, and National Museum for Wales sites were closed to the public.

The Rail Maritime and Transport union said work on Royal Fleet Auxiliary ships in port in Birkenhead and Portland had been halted by the strike.

Union leaders warned the industrial unrest was fuelled by ministers making clear in Wednesday's Queen's Speech that they were pressing ahead with their controversial reforms.

Francis Maude, the Cabinet Office Minister, described the strike as "futile" and insisted that talks over pensions will not be reopened.

The walkout follows last November's huge stoppage by more than one-and-a-half million workers in protest at the changes to their pensions.

Most public sector unions remain opposed to the reforms, which they warned would leave millions of workers having to pay more into their pensions, retiring later and receiving less when they stop work.

But FDA members in senior grades of the civil service voted by almost 3-1 to accept the deal negotiated with the Government for new pension arrangements from 2015.

"This result should not be interpreted as an endorsement by FDA members of the new scheme," said Dave Penman, the Deputy general secretary.

"FDA members remain deeply unhappy about aspects of the changes and the Government's approach to pension reform."

Up to 16,000 off-duty police officers will don black caps representing each officer expected to be lost under the Government's budget cuts as they take to the streets today.

The officers, banned from striking under law, will march through central London in a protest against proposed changes to their pay and conditions.

Gail Cartmail, the Unite assistant general secretary, added: "Today's industrial action will build on the high level of anger that was on display during the November 30 strikes.

"This anger has been increased by the Government's hardline insistence that public sector employees work longer, pay more and receive less when they eventually retire.

"Our members believe that the Government is attacking their pensions as a means of helping reduce the budget deficit, which has been caused by a greedy City elite, that has brought the economy to its knees. This is blatantly unfair.

"George Osborne's austerity plans are beginning to sicken everyone. A work-until-you-drop culture in this country is not because people want teachers, nurses, firemen struggling at work into their 70s."

The pensions dispute has been raging for more than 18 months, with warnings of further strikes next month and throughout the summer.

Ministers insist the current level of public sector pensions is unsustainable and reforms are needed, saying workers will still receive decent payments on retirement.

Mark Serwotka, general secretary of the Public and Commercial Services union, said: "The coalition has reduced tax for super-earners while making cuts to vital public services like jobcentres, borders and tax collection.

"Public sector workers have seen thousands of their colleagues sacked, their pay has been frozen for two years and they are being told they must pay much more and work for up to eight years longer for smaller pensions.

"That's why hundreds of thousands of workers will be striking in opposition to the Government's prescription of austerity and misery that has plunged the UK back into recession."

Immigration Minister Damian Green said: "This strike is completely unnecessary and we believe the public will find it unacceptable.

"The security of the UK border is of the utmost importance and we will use contingency plans to ensure we minimise any disruption caused by planned union action. We are preparing to use our trained pool of backroom staff and MoD police to boost staffing levels at ports and airports around the UK."

Mr Maude added: "It is very disappointing that a handful of unions insist on carrying on with futile strike action which will benefit no one.

"We would urge these union leaders to reconsider their position. Pension talks will not be reopened and nothing further will be achieved through strike action.

"In March we set out our final proposed agreements on pension reform following more than a year of intensive discussions with trades unions."

He added: "Our reforms ensure that public sector pensions will remain among the very best available and that they can be sustained for the future.

"Public sector workers are being asked to work a bit longer and pay a bit more, but they will continue to get a guaranteed pension which is index-linked and inflation-proofed.

"Most staff on low and middle incomes will receive a pension at retirement as good as what they expect today, and for many it will be even better."

He contrinued: "Rigorous contingency planning is in place across all sectors to minimise the impact of the strike action and to ensure that key public services remain open.

"For example, in the civil service we expect that more than three-quarters of civil servants will be working normally."